High power gas dynamic lasers presently employ fixed geometry diffusers because they are relatively simple in design and construction and their pressure recovery performance is adequate. However, the improvement of the specific power output of gas dynamic lasers by increasing lasing cavity Mach number and decreasing cavity pressure requires diffuser performance beyond that obtainable from fixed geometry designs, which are limited to roughly "normal shock" recovery.
Variable geometry diffusers used in supersonic inlet designs can produce nearly isentropic pressure recovery, but they use massive boundary layer suction to prevent flow separation. This is possible because the pressures in an inlet are all above the external ambient pressure, so no pumping of the bleed flow is necessary. Furthermore, unlike the gas dynamic laser, there is no boundary layer build-up upstream of the diffuser.
In a gas dynamic laser diffuser, as in most wind tunnel diffusers, the static pressures are all below the external ambient pressure, so boundary layer bleed by suction requires the use of some kind of turbomachine or ejector pump. While this might be acceptable for a wind tunnel, the weight and volume of such pumps could be prohibitive for a mobile gas dynamic laser system. Boundary layer energization by injection is possible, but expensive in terms of the required mass flow rate of injectant.
Research on variable geometry wind tunnel diffusers has demonstrated that performance considerably better than that obtainable from fixed geometry diffusers could be obtained without any boundary layer suction or mass injection. Nearly twice normal shock recovery has been attained at Mach 3 and, in terms of normal shock recovery, even better performance has been achieved at higher Mach numbers.
In view of these results, it should be possible to design variable geometry diffusers for gas dynamic lasers which have significantly improved performance over present designs. However, because gas dynamic laser systems are to be mobile, diffuser designs for them must also have the lowest weight and volume possible, consistent with the desired pressure recovery. Rapid diffuser starting is also desirable, since no laser power can be extracted from the gas dynamic laser until steady supersonic flow is established in the laser cavity and working fluid from which no power is extracted is wasted mass.